Version 1.2.0 of MagicPreview has just been released bringing compatibility for MODX 3!
Also, in case you hadn’t already heard, with the latest feature-packed release of ContentBlocks, you can now use MagicPreview to preview your ContentBlocks layouts (without saving) just like you would any other resource! 
See the announcement here: ContentBlocks 1.12 now available • modmore.com